Why do CG artists often sculpt the head first?
The head is usually the focal point of a character and is therefore the most detailed part. 

The head is often sculpted separately to allow for higher subdivision/detail when sculpting, before retopologising, although with dyntopo this isn't so necessary. Many artists may also want to focus on the head to get a good feel for the character/get the character right before continuing with the body. 

It is worth noting that this isn't always the case, from what I observe its usually personal preference. If I start with just a head its usually because I was doing a quick bust and decided to turn it into a full character because I liked where it was going.